Whenever you write on your skin using Sharpie markers or other permanent markers the Sharpie and other permanent markers ink will fade off your skin on it's own within 3 to 4 days.
You can get the Sharpie off sooner by using rubbing alcohol or acetone based nail polish remover to scrub and fade the Sharpie ink or permanent marker ink off your skin.
I've always just used some rubbing alcohol and allowed it to sit on the Sharpie ink on my skin for a bit after soaking a cotton ball in it.
Then after a few minutes the Sharpie usually just comes right off the skin but sometimes it takes a few more tries.
However if you just want to wait it out and allow the Sharpie to fade naturally it will do so within the 4 days and should be fully gone after a week.
When you shower and use soap the Sharpie should start coming off your skin a little bit at a time.